Heat Production from Single Fracture Hot Dry Rock, Applications for EGS Reservoir Design

A new analytical solution for the thermal-hydraulic coupling process is derived with a 1-D steady state conductive heat flow in the body of hot rock with perpendicular water flow in the single fracture and transient heat transfer from rock to water. The heat produced from the hot rock via water flow in the idealized single fracture is demonstrated by arithmetic equations. The applicability of the analytical solution is verified by numerical calculations and is limited to conditions with fast water flow rates or high water flux and long fluid pathways. The lifetime of an EGS reservoir in these reference conditions is 23.2 years and is confined by the produced water temperature of 150 °C for commercial utilization. The heat recovery factor is 12.4%. With a power plant capacity of 5 Mw installed, the total area for extracting recoverable heat within the projected lifetime of a fracture surface of 1.58 × 106 m2 was determined. The total mass flow rate of water injected into the large fracture was 57 kg/s. The discussion shows the ability of the model to estimate heat production and reservoir scale

Effect of Vertical Permeability Heterogeneity in Stratified Formation on Electricity Generation Performance of Enhanced Geothermal System

Because geologic sedimentation and hydrofracturing processes are not homogeneous, the reservoirs of enhanced geothermal systems (EGSs) are also heterogeneous; this has a significant influence on the electricity generation performance of EGS. Presently, there are a lack of systematic and profound studies on the effect of vertical permeability heterogeneity in stratified formation on the electricity generation performance of EGS. In order to uncover the effect of vertical permeability heterogeneity on electricity generation performance of EGS, in this work we analyzed the influence of vertical permeability heterogeneity on electricity generation performance of EGS through a numerical method based on geological data at the Yangbajing geothermal field. The results indicate that when the average permeability of stratified formations is constant for a homogeneous reservoir, the system attains maximum water production rate, maximum electric power, minimum reservoir impedance and maximum pump power; with the increasing of the vertical permeability heterogeneity, the water production rate gradually decreases, the electric power gradually declines, the reservoir impedance gradually increases and the pump power gradually declines. When the average permeability of stratified formations is constant, with the increasing of the vertical permeability heterogeneity, the injection pressure and energy efficiency only changes very slightly; this indicates that the vertical permeability heterogeneity is not the main factor affecting the system injection pressure and energy efficiency

Numerical Study on the Influence of Well Layout on Electricity Generation Performance of Enhanced Geothermal Systems

The energy efficiency of the enhanced geothermal system (EGS) measures the economic value of the heat production and electricity generation, and it is a key indicator of system production performance. Presently there is no systematic study on the influence of well layout on the system energy efficiency. In this work we numerically analyzed the main factors affecting the energy efficiency of EGS using the TOUGH2-EOS1 codes at Gonghe Basin geothermal field, Qinghai province. The results show that for the reservoirs of the same size, the electric power of the three horizontal well system is higher than that of the five vertical well system, and the electric power of the five vertical well system is higher than that of the three vertical well system. The energy efficiency of the three horizontal well system is higher than that of the five vertical well system and the three vertical well system. The reservoir impedance of the three horizontal well system is lower than that of the three vertical well system, and the reservoir impedance of the three vertical well system is lower than that of the five vertical system. The sensitivity analysis shows that well spacing has an obvious impact on the electricity production performance; decreasing well spacing will reduce the electric power, reduce the energy efficiency and only have very slight influence on the reservoir impedance. Fracture spacing has an obvious impact on the electricity production performance; increasing fracture spacing will reduce the electric power and reduce the energy efficiency. Fracture permeability has an obvious impact on the electricity production performance; increasing fracture permeability will improve the energy efficiency and reduce the reservoir impedance

Numerical Study on Application Conditions of Equivalent Continuum Method for Modeling Heat Transfer in Fractured Geothermal Reservoirs

The equivalent continuum method an effective approach for modeling heat transfer in fractured geothermal reservoirs. However, presently there is a lack of systematical and profound study on application conditions of the equivalent porous media (EPM) method. In this study, we numerically investigated the application conditions of the EPM method based on geological data of Yangbajing geothermal field. The results indicate that when fracture spacing is within 3–25 m, the results of the EPM method are basically in the same levels as those of the MINC method. However, when the fracture spacing is within 25–300 m, differences of the EPM method from the MINC method increase with the fracture spacing, so when the fracture spacing is within 25–300 m, it is unreasonable to adopt the EPM method to simulate the fractured reservoirs. With the fracture spacing increasing within 25–300 m, the system production temperature and electric power will gradually decrease; the injection pressure, reservoir impedance and pump power will gradually increase; and the energy efficiency will gradually decrease

Effect of lithological rhythm on gas production performance via depressurization through a vertical well in a confined hydrate reservoir

The lithological characteristics of subsurface formations control the variations in the permeability. Herein, the effect of lithological rhythm on gas production was numerically investigated for depressurization applied in a confined hydrate reservoir. The lithological rhythm structures from simple uniform, positive, and reverse rhythms to complex composite rhythms were designed. The water-to-gas ratio showed that economic feasibility of all rhythms varied with the expansion of the hydrate dissociation region. The depth of high permeability region affected the hydrate dissociation and temperature recovery. Therefore, the gas production rate of positive rhythm was higher than that of the uniform and reverse rhythms, while the reverse rhythm was economically more suitable in the later stages of the production. Furthermore, the gas and water production rates decreased with the increase in the permeability contrast. The number of composite rhythmic layers negligibly affected the gas production rate, while it significantly affected the hydrate saturation distribution, indicating an increasing hydraulic connectivity in the lateral direction that may lead to water breakthrough. Moreover, the rhythmic structures significantly affected the gas production rate and the economic feasibility, and controlled the developed depth of high hydraulic connectivity. Therefore, minor and large seepage pathways could form in a complex rhythm reservoir

Effect of lithological rhythm of a confined hydrate reservoir on gas production performance using water flooding in five-spot vertical well system

The lithological features of sediments control the heterogeneity in permeability. This study numerically investigates the effect of lithological rhythm of a confined hydrate reservoir on gas production for warm water injection scenarios. The investigated lithological rhythm structures range from simple rhythms to complex composite rhythms. The depressurization in multiple wells causes significant gas production. However, the decrease in pressure gradient constrained the production of gas and water. The warm water injection enhances the production of gas and water with various lag times, though it results in a low energy efficiency. The positive and reverse rhythms have similar energy efficiencies. The gas and water production rate slightly decrease with the increase in the permeability contrast. The maximum gas production rate increases with the increase in number of composite rhythmic layers. However, the water production changes only slightly. The rhythm structures significantly affect the gas and water production rates. A high permeability region in the bottom part of the reservoir is conducive to warm water breakthrough. However, it is unfavorable for the dissociation of remnant hydrate in low permeability regions. Moreover, the results show that the rhythm structures also affect the gas productivity of each rhythmic layer. The results presented in this study help us understand the effect of complex and real rhythm structures on the gas production performance and the dynamic evolution of hydrate dissociation zone

Seafloor subsidence induced by gas recovery from a hydrate-bearing sediment using multiple well system

The response behavior of the methane exploitation from natural gas hydrate (NGH) using multiple well system is complex and needs to be investigated, as gas production from a single vertical well generally cannot meet commercial demand. This study numerically investigates the production performance and geomechanical response of an unconfined hydrate deposit in Shenhu area, South China Sea, under single and multiple vertical well conditions. For a single vertical well with a mild constant bottom-hole pressure, gas production is relatively stable. However, seafloor subsidence exhibits an initial rapid drop and a subsequent mild drop stage. The vertical displacement is highly developed at the top and bottom of the production zone. The results from doublet and triplet vertical wells indicate that both the gas and water production and seafloor subsidence increase with the increase in number of production wells. The superimposition of subsidence leads to a deterioration in subsidence and the change in location for the largest subsidence, which may affect the risk location of well instability. The interference of pore pressure and subsidence increases with the decrease in well spacing. However, gas production decreases and water production changes insignificantly. Furthermore, a same subsidence at seafloor cannot indicate the same evolution of subsidence in the vertical and lateral direction. The results presented in this study help in balancing the production and subsidence of the NGH exploitation in complex well configurations